This invention relates to a lens array used for an image transmitter such as, e.g., an electronic printer, a facsimile machine, and a scanner, to an exposure device adopting the lens array, and to an image forming apparatus furnished with the exposure device.
Lens arrays (or rod lens arrays) in which a plurality of minute rod lenses in a cylindrical shape are arrayed are used as lens for an image transmitter built into an apparatus such as, e.g., an electronic printer, a facsimile machine, and a scanner. A rod lens is an optical lens made of a cylindrical-shaped resin or glass material with a refractive-index distribution in a radial direction. A lens array is an optical component in which the plurality of rod lenses are arrayed and unified, and is used as one part of an image sensor of a photocopier, the scanner, or the like. The lens array is also used as an exposure device forming an exposure image on a photoreceptor at a high resolution in combination with a luminous source such as an LED (Light Emitting Diode) in various apparatuses and particularly in electrophotographic systems.
In recent years, since high quality improvement is required for formed images, the exposure device is also required to improve resolution to around, e.g. 1200 dpi in order to achieve improvement in image quality. As a technique for improving the image quality, such a rod lens has been widely known whose refractive-index decreases continuously from a central axis to an outer periphery, and which is provided with a light absorbing layer having a thickness of 50 microns or greater in a range outside the range of 0.6 of the radius of the rod r or greater from the central axis of the rod lens. The light absorbing layer contains a light absorbent for absorbing at least part of the light transmitted through the rod lens (see, e.g., Japanese Patent Laid-Open Publication No. 2003-114306).
However, in a case of formation of an exposure image at a high resolution on a photoreceptor by the aforementioned conventional technique, the exposure image has insufficient contrast between a bright section and a dark section. For example, where the image forming apparatus forms an image at 1200 dpi or higher resolution, i.e., a resolution of 1200 dots per inch, the image quality deteriorates in terms of granular quality or a blur of a thin line in a halftone image, and the like. By the method for forming the light absorbing layer described in Japanese Patent Laid-Open Publication No. 2003-114306, it is not easy to form the light absorbing layer having a predetermined micron-size thickness that uses a light absorbent such as a dye. The resulting variation in thickness directly leads to deterioration in optical characteristics.
In consideration of the foregoing technical problems, this invention is intended to provide a lens array which can prevent deterioration in image quality even in a case of forming the image at high resolution, and allow easy manufacturing with the stable optical characteristics.